The C.sub.4 hydrocarbon fraction which is obtained by the thermal or catalytic cracking of mineral oils is of particular commercial interest because it contains valuable raw materials. Depending on the intensity of cracking especially thermal cracking, the C.sub.4 streams contain different quantities of butadiene, n-butylenes, isobutylene, acetylene compounds and saturated hydrocarbons. Inter alia butadiene is the starting product for the production of synthetic rubber and a large group of thermoplastic materials. n-butenes are the starting products for many commercially significant syntheses, for example for synthesizing secondary butanol, methylethylketone and butadiene. The C.sub.4 streams are also the feedstock for alkylation plants in which they are converted into anti-knock fuels.
In order to efficiently carry out each of the above-mentioned further processes, it is necessary that the isobutene be removed quantitatively as far as possible. None of the known methods achieves such an extensive removal of isobutylene in the presence of butadiene. Isobutene can be separated from butadiene-free, C.sub.4 streams down to approximately 0.3% but this involves the loss, as by isomerization, of substantial quantities of n-butylenes.
A conventional method for the removal of isobutene from C.sub.4 streams is the etherification of this hydrocarbon with an alkanol containing 1-4 C-atoms in the presence of an acid ion exchanger as catalyst, because this reaction is selective for olefins with a tertiary C-atom adjacent to the double bond. Methanol is advantageously used as alkanol, since the methyl, tert-butyl ether formed therefrom is a valuable additive for producing anti-knock properties in gasolines.
The reaction of isobutene with a lower alkanol, such as methanol, is an equilibrium reaction. It is known that equilibrium reactions can be shifted to the side of the product if an excess of one of the two reaction partners is used. If isobutene is to be removed from the C.sub.4 stream it would therefrom appear to be merely necessary to use an excess of methanol. However, this suffers from the disadvantage that the methyl, tert-butyl ether must be washed for removing the methanol which is a labour-intensive procedure and leads to a loss of ether. A method has therefore been developed in which the quantity of methanol used is equi-molar with the isobutene. The German Offenlegungsschrift No. 25 21 963 can be used as an example. This method operates in two stages in which an excess of isobutene is added in one stage and an excess of methanol is added to the other stage. The reaction is performed with an acid ion exchanger as catalyst. A relatively high flow velocity is required to avoid oligomerization of the isobutene and etherification of the butadiene and similar side reactions. Space velocities of 20 to 50 vol/h are recommended in the above-mentioned German Offenlegungsschrift. Quantitative etherification of the isobutene is of course impossible with such a high flow velocity. If the space velocity is reduced, approximately 3 to 10% of undesirable products will be produced, depending on the butadiene content of the output stream.